1. Field of the Invention
The present invention relates to an image developer used in electrophotographic methods for copiers, printers and facsimiles, etc., and more particularly to a non-magnetic one-component image developer capable of both applying a toner onto a developing roller and scraping the toner therefrom. In addition, the present invention relates to a process cartridge and image forming apparatus using the image developer.
2. Description of the Related Art
An image developer in an image forming apparatus rotates a developing roller bearing toner to feed the toner to a place facing a photoreceptor as an image bearer, and develops an electrostatic latent image formed on the surface of the photoreceptor with the toner.
It is important to form a thin toner layer on the developing roller to produce high-quality images, and various methods of forming the thin toner layer are disclosed.
Japanese published unexamined application No. 54-43038 (JP-S54-43038-A) discloses a method of contacting an elastic rubber or metallic blade to a developing roller, passing toner through a gap therebetween to form a thin toner layer on the developing roller, and charging the toner.
Therefore, a toner feeder is needed to feed a required quantity of toner onto the developing roller. Methods of contacting a toner feeding and collection roller formed of a brush material or a foamed rubber material to the developing roller to apply the toner thereto are known.
However, besides a capability of feeding a toner thereto the toner feeding and collection roller also has a collection capability of scraping off undeveloped toner remaining on the developing roller after image development. When this capability is insufficient, the result is that charged and undeveloped toner and uncharged toner are mixed together on the developing roller. The difference in charge between the two types of toners causes a difference in the resultant image density, i.e., production of ghost images.
Methods of electrostatically scraping off toner are known, in which the toner feeding and collection roller is formed of a semiconductive foamed elastic material and a DC bias voltage having a polarity opposite to that of the charged toner is applied to the roller to increase the ability to scrape away the toner.
However, the DC bias voltage not only increases the capability of scraping away toner but also disturbs the capability of applying toner to the developing roller in the first place. Therefore, when images having high image density are continuously produced, the supply of toner fed onto the developing roller becomes inadequate, resulting in production of images having lower image density and blurred images.
Japanese published unexamined application No. 2001-249532 (JP-2001-249532-A) discloses a non-magnetic one-component image developer in which an AC bias voltage is applied to a toner feeding and collection roller formed of two foamed rubber layers having different volume resistivities for the purpose of preventing ghost images by replacing an uncharged toner on the feeding and collection roller with an undeveloped toner remaining on a developing roller. This method oscillates the toners in an alternating electric field formed between the developing roller and the feeding and collection roller to alternate the toners.
However, the AC bias voltage needs to have a desired waveform, and for this, the feeding and collection roller needs to include a lower layer having a volume resistivity not greater than 105 Ω·cm and an upper layer having a volume resistivity not less than 106 Ω·cm. Therefore, it is difficult to control the volume resistivities, resulting in higher cost.
To prevent ghost images from being produced, the AC bias voltage must be applied to the feeding and collection roller, and its volume resistivity is just a supplementary condition for effecting the AC bias voltage. However, the AC bias voltage causes cyclic uneven image density because the adherence of toner onto a developing roller changes according to the oscillation cycle, and an electrical source for generating the voltage is disadvantageously expensive.
Meanwhile, a DC bias voltage does not solve the problem.
Japanese published unexamined application No. 05-333679 (JP-05-333679-A) discloses an image developer in which a toner feeding roller and a toner collection roller are separately located, and a bias voltage feeding a toner to a developing roller is applied to the toner feeing roller and a bias voltage scraping the toner therefrom is applied to the collection roller to provide both feeding and collection capabilities. However, a configuration in which plural rollers contact each other around a developing roller is complicated and difficult to downsize.
For these reasons, a need exists for a downsizable image developer having a simple configuration.